In the field of this invention it is known to use direct conversion or zero-IF receivers in radio receivers for applications such as cellular telephony. In such receivers, it is necessary to maintain the spectral purity of the channel used for reception. Because of limited narrow band selectivity, second order intermodulation distortion (IM2) presents an undesired spectral component within the signal band of interest. This occurs when two or more interfering signals, whose difference in frequency is less than the IF bandwidth of the desired signal, mix with one another due to some second order nonlinearity and produce a baseband spectral component. To minimize the effects of second order intermodulation within critical circuit blocks in the signal path, it is known in the art to use differential circuits. In theory, differential circuits have infinite attenuation to second order intermodulation distortion; however, in reality this is far from the truth, due in no small part to device mismatches, parametric imbalance, imperfect layout, and other device characteristic inequalities that cause imbalances which provide a lower than desired second order input intercept point (IIP2). As will be appreciated by those skilled in the art, the best IIP2 achieved to date in the integrated mixer art may fall significantly short of system requirements. It would be extremely advantageous, therefore, to provide improved chopping mixer performance so as to allow the above difficulties to be overcome. It would be of greater advantage to apply this improved chopping mixer performance to wireless and wireline communications, devices that employ mixer circuits, switches, and other components that exhibit parametric mismatch or imbalance.
United States patent U.S. Pat. No. 5,859,559 (RAYTHEON) describes a mixer structure suitable for inclusion as part of an integrated circuit. Spurious signals is avoided by introducing trickle currents which enhance the transconductance of an input differential amplifier.
United Kingdom Patent Application No. GB-A-2 151863 (Toshiba) describes an amplifier circuit having first and second differential amplifiers. A switch circuit enables the dynamic range of the output signals to be increased. First and second outputs are applied to a load so as to obtain the product of the first and second signals.
It is an object of the present invention to provide method and apparatus for improving chopping mixer performance wherein the abovementioned disadvantage(s) may be alleviated.